3145.     ---------------.  [SCHWARTZ, H. A.]  Conversion of Solid Cementite Into Iron and Graphite.  Jour. Iron Steel Inst., vol. 138, 1939, pp. 205P-240P; Chem. Abs., vol. 32, 1938, p. 9012.

        Our knowledge of the heat, entropy, and free energy changes accompanying the reaction Fe₃C=3Fe+C are still unsatisfactory owing to the imperfections in the best existing work on the underlying data.  Evidence is offered regarding the possibility of the existence of 2 types of solid solutions and the relationship of this possibility to the constitutional diagram, especially the A₃, or G’S’, line of the double diagram.  The graphitizing reaction involves the formation of graphite nuclei, the solution of cementite in Fe, the dissociation of cementite into Fe and C, the migration of C either before or after dissociation, and the crystallization of graphite.  The reaction rate is shown to be controlled by the migratory rate, this being the slowest step except at the very beginning and end of the process.  The graphitizing rate is thus, in fact, mainly a function of the nodule (nucleus) number and migratory rate.  The nodule number is determined largely by the prevalence of interfaces in the metal at the graphitizing temperature.  The number of interfaces depends on the fineness of the original white-Fe structure, which in turn is a function of the cooling rate.  Supercooled white cast Fe, the matrix of which is hypereutectoid in composition, may carry additional interfaces into the graphitizing temperature range.  For a given set of interfaces the nucleus number is a function of the graphitizing temperature, the O content of the metal, migratory rate of C is influenced mainly by the presence of other alloying elements.  Certain elements present in atomic solution or as chemical compounds appear to retard the migratory rate, whereas a few accelerate graphitization, probably by increasing the migratory rate.  The effectiveness of the elements of group VI of the periodic table on the migratory rate and the fact that the effectiveness of elements in a given group often decreases with increase of the atomic weight may be of interest from the electronic viewpoint.  64 refs.